WSUS Storage Capacity Sugestions

I am installing a WSUS server in the school system I work for. I am very new to this setup. I have been reading the Windows Server Update Servcies 3.0 SP2 Deployment Guide. I am trying to decide on a reasonable amount of HDD space to allocate for storing the Updates. We are useing the following environment.

We will have an Upstream Server at our Central Office that will pull the necessary updates from Microsoft. This Upstream Server will then Replicate these updates to a Downstream Server located at each school.